She met the young barrister Arthur Llewelyn Davies at a dinner party in 1889 and they became engaged shortly thereafter.[1] She married him in 1892, and they had five children, all boys: George (1893–1915), Jack (1894–1959), Peter (1897–1960), Michael (1900–1921), and Nicholas (Nico) (1903–1980).

In 1898, Davies met Barrie at a dinner party, discovering he was already friends with her three sons from their regular visits to Kensington Gardens. She and Barrie became close (he adopted the pet name "Jocelyn" for her), with him spending considerable time at the Davies's home, and the family accompanying Barrie and his wife on holidays. She encouraged her boys' friendship with him.

Her husband died in 1907 of a sarcoma in his cheek. Davies welcomed Barrie's financial and emotional support, both for herself and for her boys. Following Barrie's divorce, he and Sylvia remained close, but did not marry. She became ill with an inoperable cancer in her chest, and died in 1910. Shortly before her death, she wrote that she wanted her boys' nurse Mary Hodgson to continue caring for them, and that she knew Barrie would continue providing for them, which he did. She named him, along with her mother Emma du Maurier, her brother Guy du Maurier, and Arthur's brother Crompton Llewelyn Davies as their guardians. Barrie told the boys after her death that she had been engaged to him, but Jack and Peter later expressed scepticism of this report.

Her son Peter was the publisher of her niece Daphne du Maurier's book about their grandfather, The Young George du Maurier, letters 1860–1867 (1951).